Many barriers exist to minimizing the form factor of optical disk drives. For example, conventional optical disk drives such as a CD-ROM drive are configured for use with “second surface” optical disks. In a second surface optical disk, the information layer is covered by a relatively thick protective layer or substrate that is hundreds of microns in thickness. Considering that conventional laser light used to read and write in optical drives has a wavelength in the range of from around 400 to 800 nanometers, the relatively thick protective layer is thus many wavelengths in thickness. As such, imperfections such as scratches, dust, and fingerprints on the surface of the protective layer are defocused with respect to the underlying information layer. In this fashion, CD-ROMs and other second surface disks may be handled by users and exposed to dusty environments without needing a protective cartridge.
Although the use of second surface disks provides this advantageous defocusing property, it is also accompanied by certain drawbacks. For example, the relatively thick protective layer covering the information layer introduces significant optical aberrations and wave front distortions. In turn, these optical problems place a floor on the achievable feature size in the information layer, thereby limiting data capacity. However, as the optical disk size is reduced, it is important to minimize feature size in the information layer to provide significant data storage capability despite the presence of a relatively small information layer area. To achieve a significant data capacity within a small form factor optical disk drive, the present assignee has developed first surface optical disks such as disclosed in U.S. Ser. No. 10/891,173, filed Jul. 13, 2004, which is a divisional application of U.S. Ser. No. 09/315,398, filed May 20, 1999, now abandoned, the contents of both applications being incorporated by reference herein in their entirety. In these first surface disks, an information layer covers a substrate, which may be formed to define one or both of a read-only and a writeable area. Advantageously, the information layer may be formed from a continuous phase-change material such as an SbInS alloy so that the formation of the read-only and writeable areas (if both exist) requires no masking or other complicated manufacturing processes. The surface of the information layer may be covered with an optical coupling layer formed from a dielectric such as silicon oxynitride. However, unlike a second surface disk, the optical coupling layer is quite thin (such as less than 100 nm) thereby providing no defocusing effect. Instead, it merely functions as an anti-reflective coating as known in the optical arts to better optically couple the information layer to an air-incident laser beam. Advantageously, the optical coupling layer does not introduce the aberrations and wave front distortions that the protective layer in second surface optical disks does such that the feature size may be substantially reduced. In this fashion, a significant data capacity is achieved despite the presence of a small form factor.
The present assignee also developed a small form factor optical disk drive for use with the inventive first surface optical disks. For example, U.S. Ser. No. 09/950,378, filed Sep. 10, 2001, discloses an optical disk drive having an actuator arm with an optical pick-up unit (OPU) mounted on one end. A cross-sectional view of an OPU 103 is shown in FIG. 1a. A corresponding optical ray trace diagram for OPU 103 is illustrated in FIG. 1b. As seen in FIG. 1a, OPU 103 includes a periscope 210 having reflecting surfaces 211, 212, and 213. Periscope 210 is mounted on a transparent optical block 214. An object lens 223 is positioned on spacers 221 and mounted onto quarter wave plate (QWP) 222 which in turn is mounted on periscope 210. Optical block 214 is mounted through turning mirror 216 and spacer 231 to a silicon submount 215.
A laser 218 is mounted on a laser mount 217 and positioned on silicon submount 215. Detectors 225 and 226 are positioned and integrated onto silicon substrate 215. Laser 218 produces an optical beam 224 which is reflected into transparent block 214 by turning mirror 216. Beam 224 is then reflected by reflection surfaces 212 and 213 into lens 223 and onto an optical medium (seen in FIG. 1b). In some embodiments, reflection surfaces 212 and 213 can be polarization dependent and can be tuned to reflect substantially all of polarized optical beam 224 from laser 218. QWP 222 rotates the polarization of laser beam 224 so that a light beam reflected from the optical medium is polarized in a direction orthogonal to that of optical beam 224.
A reflected beam 230 from optical medium 102 is collected by lens 223 and focused into periscope 210. A portion (in some embodiments about 50%) of reflected beam 230, which is polarized oppositely to optical beam 224, passes through reflecting surface 213 and is directed onto optical detector 226. Further, a portion of reflected beam 230 passes through reflecting surface 212 and is reflected onto detector 225 by reflecting surface 211. Because of the difference in path distance between the positions of detectors 225 and 226, detector 226 is positioned before the focal point of lens 223 and detector 225 is positioned after the focal point of lens 223 as seen in FIG. 1b. 
In some embodiments, optical surface 212 is nearly 100% reflective for a first polarization of light and nearly 100% transmissive for the opposite polarization. Optical surface 213 can be made nearly 100% reflective for the first polarization of light and nearly 50% reflective for the opposite polarization of light, so that light of the opposite polarization incident on surface 213 is approximately 50% transmitted. Optical surface 211 can, then, be made nearly 100% reflective for the opposite polarization of light. In that fashion, nearly 100% of optical beam 224 is incident on optical media 102 while 50% of the collected return light is incident on detector 226 and about 50% of the collected return light is incident on detector 225. A portion of laser beam 224 from laser 218 can be reflected by an annular reflector 252 positioned in periscope 210 on the surface of optical block 214. Annular reflector 252 may be a holographic reflector written into the surface of optical block 214 about the position that optical beam 224 passes. Annular reflector 252 reflects some of the laser power back onto a detector 250 integrated onto silicon submount 215. Detector 250 provides an Automatic Power Control (APC) signal that can be used in a servo system to control the output power of laser 218.
Turning now to FIG. 2, an exemplary actuator arm 104 is illustrated. Actuator arm 104 includes OPU 103 at one end. By rotating about an axis B through a spindle 200, actuator arm 104 may move OPU 103 radially with respect to an optical disk (a portion of which is illustrated in FIG. 1b). As used herein, radial movement is defined as movement parallel to an optical disk surface. Thus, to maintain tracking of an optical disk by OPU 103, a tracking servo will command a desired radial displacement of actuator arm 104. By flexing actuator arm 104 about an axis A, OPU 103 may move axially with respect to an optical disk to achieve a desired focus. As used herein, axial movement is defined as movement transverse to an optical disk surface. Thus, to maintain focus, a focus servo will command a desired axial displacement of actuator arm 104. By providing an actuator arm having these properties, a small form factor optical disk drive may be implemented. For example, the height of a disk drive incorporating OPU 103 may be as little as 10.5 mm. However, note that OPU 103 is aligned such that its height dimension H is normal to or in the axial direction with respect to an optical disk surface. Thus, the overall achievable height reduction of such a drive architecture is limited by the thickness of the optical disk and its cartridge as well as height H of OPU 103 (as measured from the bottom of OPU to the focused laser spot at the disk surface).
In certain applications such as mobile phone handsets, is desirable to provide an optical disk drive having a thickness of as little as five mm. However, in the disk drive architecture discussed with respect to FIG. 1a through FIG. 2, the addition of the disk cartridge thickness to the height of the OPU makes the achievement of such a small form factor problematic.
Accordingly, there is a need in the art for optical disk drives and corresponding optical media configured to achieve small form factors.
In addition, the use of a split-optics-based architecture in a optical disk drive introduces “beam walk” resulting from the motion of the laser beam on the photodetectors. The beam walk introduces an offset into the tracking error signal such that a tracking offset is introduced into the TES (Tracking Error Signal) that for the small lens pupils in small form factor optical devices rapidly becomes large and causes the tracking servo to track off the center of the track. Conventional approaches to solve the beam walk problem include the use of a three beam OPU. However, the use of a three beam OPU increases manufacturing costs and complexity. In addition, optical efficiency is reduced because laser power must be diverted to form the additional beams and additional alignment criteria are involved for the additional spots.
Accordingly, there is a need in the art for improved beam walk compensation approaches.